Nanocomposite fibres: a strategy for stronger materials?

The scientific literature is full of claims for the exceptional potentialities of nanoscale reinforcing materials, such as nanoparticles, nanofibres, nanotubes and nanoplatelets, to improve the mechanical properties of polymer matrices. However, most of the reliable data on the effective properties of polymer nanocomposites are somewhat disappointing, particularly when compared to structural composites reinforced with high-performance continuous fibers. The main reasons advocated to explain the discrepancies with theoretical predictions include poor dispersion, inadequate alignment of nanofillers, and bad stress-transfer ability at the interface (i.e. poor adhesion with polymer matrix). A possible route for introducing nanofillers into polymer matrices, and retaining a certain control on orientation and improving the dispersion level, can be offered by polymer fibres technology. In fact, the elevated draw ratios and elongational flow involved in typical fibre production processes can concurrently promote a disruption of the aggregates and a strong orientation of the nanofillers along the fibre axis. A variety of processing techniques can be adopted for the production of nanocomposite fibres, including spinning from polymer melts or solutions, gel-spinning, melt-blowing and electrospinning. This approach has been proven to be extremely efficient when nanofillers with an elevated aspect ratio, such as nanofibres or nanotubes, are concerned. In fact, in the last decade several papers have been published documenting the extraordinary reinforcing efficiency of single- or multiwalled carbon nanotubes in highly oriented polymer fibres or tapes. More recently, some successful attempts have been made to extend this approach to nanoplatelets, in particular layered silicates and graphene. Among the drawback to overcome, one can list the marked viscosity increments generally induced by nanofillers in polymer matrices, and the consequent necessity to identify proper processing conditions for the fibre production. Moreover, the decrease of elongation at break (which is also reflected in the molten state) typically induced by some nanofillers may also limit fibre preparation possibilities. The outstanding properties of nanocomposite fibres could improve traditional textile products, or be exploited in some advanced processing technologies, such as single- or all-polymer composites and commingled yarns composites

Improving fibre/matrix interface through nanoparticles

strength controls several mechanical properties of composite materials, in particular the matrix-dominated ultimate parameters. In the last thirty years an impressive number of experimental and modelling efforts have been focused on the understanding of fibre/matrix interfacial bond with the aim to improve it. Over the years, two main strategies emerged for polymer composites: i) the development of specific fibre sizings/coatings/treatments and/or ii) the addition of coupling agents to the matrix resin. In the recent years new strategies came to light: in fact, the availability of various types of nanoparticles offered the possibility to tailor the fibre/matrix interactions at a nanoscale level. In particular, some recent investigations proved that nanoparticles homogeneously dispersed in a polymer matrix can play a beneficial role on the fibre/matrix interfacial adhesion in several types of structural composites. For example, the introduction of organo-modified clays in an epoxy matrix led to the formation of a stronger interface with E-glass fibres, with an increase of the interfacial shear strength of about 30% for a filler content of 5 wt% (DOI: 10.1177/ 0021998311420311). Concurrently, the evaluation of the fibre/matrix contact angle revealed an improved wettability when organo-modified clays were added, and a simultaneous enhancement of the fracture toughness of the resin matrix. This approach could be also adopted to add specific functional properties to composite materials, such as damage controlling capabilities (DOI: 10.1016/j.compositesa.2012.03.019). Another approach relies on the possibility to confine nanoparticle in the interfacial region, with the advantage of localizing their presence in the area where stress transfer takes place, thus reducing the overall quantity of nanoparticle required. As an example, a sizing containing single or multi-walled carbon nanotubes has been used for coating glass fibres (DOI: 10.1016/j.compscitech.2007.10.009). Two simultaneous results have been reached, to 'heal' surface flaws and to enhance interfacial adhesion in a polypropylene matrix, indicating nanotube related interfacial toughening mechanisms. An amazing amount of research has gone, and still goes, into the understanding of the properties of nanoscale particles and their usage to improve engineering materials. Development of polymer composites can surely benefit from this research, including the 'old issue' of fibre/matrix interface

The way to autonomic self-healing polymers and composites

composites may cause irreversible damages such as surface scratches, micro cracks or small internal flaws that can seriously impair the load carrying ability of structural components. In fact, these cracks can grow under subcritical loading conditions (such as fatigue or creep) and ultimately lead to failure. Repairing these small damages is often a difficult task since it requires non-destructive inspections an intervention under maintenance procedures. A big challenge for polymer scientists and engineers is to develop new polymeric matrices having the built-in ability to partially repair damages occurring during its service life time. An ideal self-healing polymer is able, after having been damaged, to autonomously restore its initial mechanical performances without the need of external stimuli like temperature, radiation, pressure, etc. This process is quite common in nature, since damage to a living tissue generally elicits a healing response. Self-healing applied to thermosetting matrices and composites is an emerging area of research, with the first significant papers being published in early 2000's and with the 1st International Conference on Self-Healing Materials organized by the Delft University of Technology (NL) in 2007. The first report of a man-made self healing polymer was by the group of prof Scott White of the University of Illinois at Urbana-Champaign. They developed an epoxy system containing microcapsules filled with a healing agent (liquid monomer). When a crack propagates, the microcapsule will rupture, the monomer will fill the crack and eventually undergo a polymerisation process, initiated by (Grubbs') catalyst particles dispersed in the system. This model system proved to work quite well, as the service life time of such material under fatigue conditions is significantly improved. On the other hand, it also presents a number of drawbacks related to the Grubbs' catalyst (high cost, partial deactivation by the amine curing agent, decomposition above 120°C) and to the stability of the microcapsules (diffusion and leakage of the healing agent, decomposition of the capsules above 170°C). A number of research groups world wide is trying to improve the microencapsulation-based approach or to develop new approaches to self-healing. A possible alternative strategy is based on the development of reversible crosslinked polymers through selfassembly chemistry in which the material is repaired by the re-association of the self-assemblable bonds broken by the crack. At a present, there are no commercially available products having a self-healing ability and the challenge for materials scientists and engineers is fascinating

"Or ti riman, lettor, sovra 'l tuo banco". Il ms. Egerton 943 della British Library

Il lavoro si configura come un'indagine interdisciplinare di uno dei primi e più importanti testimoni della "Commedia" di Dante, il ms. Egerton 943 della British Library. Di fronte alla complessità del prodotto librario si è scelto di operare sul piano della critica testuale per i problemi legati al testo e al commento (il cosiddetto 'Anonimo latino'), sul piano dell’indagine storica per il collocamento temporale e geografico del manoscritto, su quello dell’iconografia e dell’iconologia nell’esame delle immagini e dei diagrammi di inferno e paradiso

An agent-based model of product competition: network structure and coexistence under different information regimes

The paper analyzes how the structure of interaction networks affects the diffusion patterns and market shares of different products in case of local network externalities and imperfect information. The diffusion of the different products/technologies in the market is modelled as the result of two (only partly) interrelated dynamics: i) the interaction between idiosyncratic individual thresholds and local network externalities; ii) the diffusion of the information about the product (via broadcast diffusion and word-of-mouth). The average clustering coefficient affects the overall outcome and the actual possibility that one product corners the market. Moreover, in case of small-world networks, despite the high clustering coefficient which increases the probability of an outcome with coexistence, the increase in the speed of diffusion impinges on the actual realization of such an outcome in case of sequential entry of the different technologies and/or imperfect information.Agent-based model, innovation diusion, network eects,social networks, small-world

Proposal of the Boltzmann-like superposition principle for nonlinear tensile creep of thermoplastics

Abstract The Boltzmann superposition principle (BSP) valid for "standard linear solids" is presented in all textbooks on viscoelasticity. In practice, the BSP is not applicable to viscoelastic polymers because (i) the apparent limit (if any) of the stress–strain linearity is very low, (ii) real deformations (stresses) are not infinitesimal, and (iii) tensile deformations give rise to additional free volume, which affects all currently running deformation processes. Consistent application of the free volume approach, including the strain-induced free volume, allowed us to derive and verify a new type of the internal time–tensile strain superposition for a series of single-step nonlinear creeps [J. Kolařik, A. Pegoretti, Nonlinear tensile creep of polypropylene: time–strain superposition and creep prediction, Polymer 47 (1) (2006) 346]. The Boltzmann-like superposition principle for multistep nonlinear tensile creep, proposed in this paper, consists of (i) the separation of individual creeps, (ii) their reconstruction for the initial free volume by introducing a specific internal time, and (iii) the superposition of the reconstructed creeps. The procedure is demonstrated using creep data for three types of commercial polypropylene

On the toughness of thermoplastic polymer nanocomposites as assessed by the essential work of fracture (EWF) approach

The essential work of fracture (EWF) approach is widely used to determine the plane stress fracture toughness of highly ductile polymers and related systems. To shed light on how the toughness is affected by nanofillers EWF-suited model polymers, viz. amorphous copolyester and polypropylene block copolymer were modified by multiwall carbon nanotube (MWCNT), graphene (GR), boehmite alumina (BA), and organoclay (MMT) in 1 wt% each. EWF tests were performed on deeply double-edge notched tensile-loaded specimens under quasistatic loading conditions. Data reduction occurred by energy partitioning between yielding and necking/tearing. The EWF prerequisites were not met with the nanocomposites containing MWCNT and GR by contrast to those with MMT and BA. Accordingly, the toughness of nanocomposites with homogeneously dispersed and low aspect ratio fillers may be properly determined using the EWF. Results indicated that incorporation of nanofillers may result in an adverse effect between the specific essential and non-essential EWF parameters

Nutrient Removal in Willow Biomass Crops is Impacted Over Multiple Rotations, Timing of Harvest, and Harvesting System

The pressing need to mitigate climate change and find alternative uses for marginal agricultural land have stimulated the establishment of short rotation woody crops (SRWC), like shrub willow, in both North America and Europe. There is limited research on the dynamics of nutrient removal over several rotations in these systems and little is known about the long-term impacts of repeated whole-plant harvesting on soil nutrient concentrations. This study compared nutrient removals among 18 cultivars of willow harvested across three three-year rotations at two sites and changes in the soil nutrient concentrations. Nutrient removal was statistically different among rotations for all studied elements in the following order 2011 ≤ 2017 \u3c 2014. For example, K removal was 7 kg ha-1 year-1 in 2011, 14 kg ha-1 year-1 in 2017, and 20 kg ha-1 year-1 in 2014 at the Belleville site. Additionally, significant effects of site (for N and Ca) and cultivar (all elements) were observed. A significant decrease in soil concentrations among years was observed for total N (1,986 g kg-1 in 2008 and 1,633 g kg-1 in 2017) and P (6.9 g kg-1 in 2008 and 3.4 g kg-1 in 2017) at one site (Belleville) while a significant increase was observed for K (44 g kg-1 in 2008 and 57 g kg-1 in 2017) at the other site (Tully). These results show that shrub willow crops are not negatively impacting extractable nutrient reserves and are capable of recycling nutrients effectively over a 10-year period. Adequate nutrient management guidelines for commercial willow sites should be site specific, consider the selection of cultivars deployed given the high variation in nutrient removal among cultivars, and the soil nutritional status